1. Field of the Invention
The present invention relates to a converging solar battery element, and more particularly to a converging solar cell element having an improved light receiving surface in which electrodes are formed on the rear surface of a silicon substrate.
2. Description of the Related Art
Solar cells are used for a number of applications and there have been proposed many different types of converging solar cell modules in which sunlight is converged by means of a lens system so that the total area of expensive solar cells can be reduced in order to reduce the cost of electric power generating systems using these solar cells. In addition, various sun tracking systems have been proposed to enhance the power generating efficiency of the converging solar cell module.
In such converging solar cell modules, converging solar cell elements each having solar cells and their electrodes for outputting electric currents are used. When a spot formed by converged sunlight irradiates the light receiving surface of the converging solar cell, free electrons and electron holes as carriers are generated inside a silicon substrate. The generated carriers are separated by a p-n junction, and the free electrons are output through an n-layer and the holes through a p-layer as currents from respective electrodes. In the conventional converging solar cell element stated above, the sun is tracked in a two-dimensional or three-dimensional direction while sunlight is converged with a converging lens. A converged spot (higher than 10 W/cm.sup.2), sometimes intensified more than 100 times, directly irradiates the light receiving portion of the solar cell. It is desirable to increase the degree of light convergence as much as possible in order to lower costs and improve the power generating efficiency in a converging solar cell module. However, if the degree of light convergence becomes too high, the size of the converged spot decreases, and the area generating the carriers and increasing a current density is reduced. As a result, internal resistance increases when the carriers move, voltage drops occur, the problem of decreased output power is created.
Also, if such a converged spot with a high degree of light convergence is generated on the light receiving surface, the surface temperature will experience sudden local rises, and cooling provided by cooling water may sometimes be insufficient. Especially, if a converged spot with an intensity greater than 18 W/cm.sup.2 is received, the temperature at the light receiving portion may rise to more than 100.degree. C., which leads to certain problems, for example, solder adhesion may decrease and the converging solar cell element may be damaged. Also, if the converging solar cell element is directly cooled with cooling water, adhesion between a mounting substrate and a sealing agent for water-proofing electrodes may be degraded, resulting in cooling water invading to the electrode side of the converging solar cell element, thereby creating a danger of electrodes corrosion or broken wires.